The present application claims priority to Japanese Patent Applications No. 2001-190225 and No. 2002-129882, file on Jun. 22, 2001 and May 1, 2002, respectively. The contents of those applications are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates a fixing device to be used in an image forming apparatus, and more particularly to a fixing device capable of preventing an excessive increase in a temperature.
2. Discussion of the Background
In an image forming apparatus, such as a copying machine, a facsimile, a printer, and other similar devices, an unfixed image that has been transferred onto a recording medium, such as a transfer sheet is fixed by a fixing device and the recording medium is discharged as a hard copy. The fixing device includes a pair of rollers provided such that the rollers oppose each other, one functioning as a heating roller, and the other functioning as a pressure roller to press a recording medium during an image fixing operation. The recording medium having an unfixed image thereon is conveyed to a nip region formed between the heating roller and pressure roller where the unfixed image is fused and fixed onto the recording medium with heat of the heating roller.
In addition, a belt-type fixing device is commonly known. An example of the belt-type fixing device includes a belt, which is spanned around a pair of rollers. One of the pair of rollers is positioned to oppose a pressure roller. The other roller of the pair of rollers drives the belt together with the roller disposed at the position opposing the pressure roller and includes a heating source inside the roller. The heating source heats the belt while the roller contacting an inner surface of the belt. The pressure roller includes a heating source inside the roller to heat an outer surface of the belt. A volume and thermal capacity of a belt is smaller than a volume and thermal capacity of a roller. Thus, a temperature of the belt increases in a shorter period of time compared to that of the roller. An advantage of the belt-type fixing device includes a shorter warm-up time as compared to the fixing device employing the heating roller and pressure roller. In addition, because a heating source is provided inside the pressure roller, the belt is heated from both inner and outer surfaces thereof, resulting in a shorter warm-up time. In the belt-type fixing device, if the pressure roller and roller with the heating source are formed of aluminum having high thermal conductivity, the belt is formed of two-layer, namely, a releasing layer and a substrate. The releasing layer includes silicone rubber or fluorine resin and is layered on the substrate including a stainless steel.
The belt is heated by the below described methods. In a first method, an inner surface of the belt is heated by a heating source included inside a heating roller, i.e., a roller that drives the belt together with the roller provided at a position opposing the pressure roller. In a second method, an outer surface of the belt is heated by a heating source included inside the pressure roller and the inner surface of the belt is heated by the heating roller. Thus, both heating and pressure rollers include a thin cylindrically shaped core metal to have a small thermal capacity. A diameter of the core metal of the heating roller is set in a range of approximately 20 mm to approximately 30 mm. A thickness of the core metal of the heating roller is set in a range of approximately 0.3 mm to approximately 2.0 mm. A diameter of the core metal of the pressure roller is set in a range of approximately 30 mm to approximately 50 mm. A thickness of the core metal of the pressure roller is set in a range of approximately 0.3 mm to 1.5 mm. Thus, the heating roller and pressure roller have a thermal capacity of not greater than 26 cal/xc2x0 C. and 36 cal/xc2x0 C., respectively.
Immediately after a recording medium such as a transfer sheet passed through a fixing region, the respective heating sources in the heating roller and pressure roller produce an excessive amount of heat when a surface temperature of the heating roller and pressure roller is increased to a predetermined set temperature. This is because an amount of heat is absorbed by the recording medium and the surface temperature of the heating roller and pressure roller is decreased below the predetermined set temperature. Hence, an excessive heating phenomenon occurs. Thus, an excessive amount of heat is supplied to a following recording medium and causes a hot offset phenomenon or produces an adverse effect such as glossiness of an image. In addition, devices for inhibiting an excessive temperature increase, such as a thermal fuse and thermostat, are damaged due to an excessive increase in temperature in the apparatus.
According to one aspect of the present invention, a fixing device includes a rotatable fixing member, a first heating device configured to heat the rotatable fixing member, a rotatable pressing member positioned in press-contact with the rotatable fixing member and forming a fixing region through which a sheet-like recording medium having an unfixed image thereon passes to fix the unfixed image by heat and pressure, and a controller configured to turn off the first heating device for a first predetermined time before a trailing edge of a last sheet-like recording medium in a series of a job passes through the fixing region.
According to another aspect of the present invention, a fixing device includes a rotatable fixing member, a first heating device configured to heat the rotatable fixing member, a rotatable pressing member positioned in press-contact with the rotatable fixing member and forming a fixing region through which a sheet-like recording medium having an unfixed image thereon passes to fix the unfixed image by heat and pressure, a second heating device configured to heat the rotatable pressing member, a controller configured to turn off the first heating device for a first predetermined time before a trailing edge of a last sheet-like recording medium in a series of a job passes through the fixing region and turn off the second heating device for a second predetermined time before the trailing edge of the last sheet-like recording medium passes through the fixing region.
According to yet another aspect of the present invention, an image forming apparatus includes a photoreceptor configured to form a latent image thereon, and a fixing device including a rotatable fixing member, a first heating device configured to heat the rotatable fixing member, a rotatable pressing member positioned in press-contact with the rotatable fixing member and forming a fixing region through which a sheet-like recording medium having an unfixed image thereon passes to fix the unfixed image by heat and pressure, and a controller configured to turn off the first heating device for a first predetermined time before a trailing edge of a last sheet-like recording medium in a series of a job passes through the fixing region.
According to still yet another aspect of the present invention, an image forming apparatus includes a photoreceptor configured to form a latent image thereon, and a fixing device including a rotatable fixing member, a first heating device configured to heat the rotatable fixing member, a rotatable pressing member positioned in press-contact with the rotatable fixing member and forming a fixing region through which a sheet-like recording medium having an unfixed image thereon passes to fix the unfixed image by heat and pressure, a second heating device configured to heat the rotatable pressing member, and a controller configured to turn off the first heating device for a first predetermined time before a trailing edge of a last sheet-like recording medium in a series of a job passes through the fixing region and turn off the second heating device for a second predetermined time before the trailing edge of the last sheet-like recording medium passes through the fixing region.
According to still another aspect of the present invention, a method of fixing an image includes providing a rotatable fixing member, providing a first heating device configured to heat the rotatable fixing member, press-contacting the rotatable pressing member with the rotatable fixing member to form a fixing region through which a sheet-like recording medium having unfixed image thereon passes to fix the unfixed image by heat and pressure, and turning off the first heating device for a first predetermined time before a trailing edge of a last sheet-like recording medium in a series of a job passes through the fixing region.
According to still another aspect of the present invention, a method of fixing an image includes providing a rotatable fixing member, providing a first heating device configured to heat the rotatable fixing member, press-contacting the rotatable pressing member with the rotatable fixing member to form a fixing region through which a sheet-like recording medium having unfixed image thereon passes to fix the unfixed image by heat and pressure, providing a second heating device configured to heat the rotatable pressing member, turning off the first heating device for a first predetermined time before a trailing edge of a last sheet-like recording medium in a series of a job passes through the fixing region, and turning off the second heating device for a second predetermined time before the trailing edge of the last sheet-like recording medium passes through the fixing region.